Heat-induced mutations in bacteriophage T4 have been demonstrated to occur by at leat two distinct mechanisms: (1) G:C base pairs are converted to A:T base-pairs (transition), (2) G:C base-pairs are converted to T:A or C:G base-pairs (transversion). The C of the base-pair is the target of the first mechanism. The G of the base-pair is the target of the second mechanism. Studies of the step(s) required for conversion of the heat-induced lesion (pre-mutational lesion) to the fully expressed mutation are continuing. The specificity of the transversion reaction (G:C to T:A versus G:C to C:G) is to be determined. T4 phage contain glucosylated hydroxymethylcytosine in their DNA. The influence of this modification on mechanism (1) will be explored.